1. Field of the Invention
The present invention relates to a system for feedback control of the air-fuel ratio of an air-fuel mixture prepared in a carburetor of an internal combustion engine based on the concentration of a specific component of the exhaust gas.
2. Description of the Prior Art
In modern internal combustion engines in automobiles, it is generally necessary to controls the air-fuel ratio of an air-fuel mixture for the engine at a predetermined value with high accuracy in order to purify the exhaust gas and reduce the specific fuel consumption. Such control is necessary for instance, with engines equipped with a catalytic converter containing a so-called three-way catalyst which can catalyze both the oxidation of hydrocarbons (HC) and carbon monoxide (CO) and reduce nitrogen oxides (NO.sub.x). The converter converts these three major noxious components of the exhaust gas into harmless substances. In such engines, it is necessary to maintain the air-fuel ratio within a very narrow range wherein there is a stoichiometric air-fuel ratio (14.7 by weight for air-gasoline mixture) because the three-way catalyst in the converter performs best when the engine is operating with a stoichiometric or nearly stoichiometric air-fuel mixture.
For controlling the air-fuel ratio, a closed loop (feedback) control system is usually used because it is difficult to attain sufficiently high accuracy of control by open loop control. A typical feedback control system for this purpose employs an exhaust sensor which detects the concentration of a specific component (for example, oxygen) of the exhaust gas, which concentration is closely related to the air-fuel ratio of an air-fuel mixture burned in the engine. The detected concentration controls the air-fuel ratio to a predetermined value, such as the stoichiometric ratio, by controlling the rate of fuel feed to the engine based on a feedback signal produced by the exhaust sensor. When the engine is equipped with a carburetor, an advantageous method of controlling the fuel feed rate is to connect an auxiliary air bleed passage to a conventional air bleed for a fuel passage in the carburetor and control the quantity of air admitted into the fuel passage through the auxiliary air bleed passage by means of the electromagnetic valve of an on-off type. An electronic control circuit supplies a constant frequency control pulse signal to periodically open and close the electromagnetic valve. The control circuit varies the duty ratio of the pulse signal based on the output signal of the exhaust sensor such that the proportion of the open-time of the electromagnetic valve to the closed-time increases when the measured air-fuel ratio indicated by the output of the exhaust sensor is lower than the desired ratio, and vice versa, whereby the air-fuel ratio converges to the desired ratio.
In practical operation of this feedback control system, however, there is an undesirable tendency that air admitted through the auxiliary air bleed passage augments significantly the air-fuel ratio when the engine is at a high speed and high load operating condition. At this condition, the flow rate of air in a venturi section of the intake passage increases. That is, the maximal value of the air-fuel ratio under the feedback control begins to gradually increase as the engine speed exceeds a certain level. This increases the width of a range in which the air-fuel ratio is controlled, making it difficult to quickly correct deviations of the air-fuel ratio from the desired ratio. As a consequence, the conversion efficiencies of the three-way catalyst are lowered considerably, particularly with respect to carbon monoxide and nitrogen oxides.
As a solution to this problem, it has been proposed to provide a variable air bleed, which is a combination of a needle valve and a vacuum-operated valve actuator of a diaphragm type. The variable air bleed is provided to the auxiliary air bleed passage such that the effective cross-sectional area of the auxiliary air bleed passage decreases as the magnitude of the venturi vacuum exceeds a certain level and becomes still greater. Such an arrangement maintains the aforementioned air-fuel ratio range practically constant irrespective of the flow rate of air in the intake passage.
However, this variable air bleed is a costly device because it is complicated in construction and has a large number of parts some of which must be produced and assembled with very high precision.